Hashimoto's thyroiditis is an autoimmune thyroid disease. The course of the disease can include hyperthyroidism, euthyroidism, and hypothyroidism. When Hashimoto's thyroiditis is at different stages of thyroid function, the changes in heart rate are also inconsistent. When Hashimoto's thyroiditis coincides with hyperthyroidism, the heart rate may be fast, even exceeding 100bpm. When Hashimoto's thyroiditis coincides with normal thyroid function, the heart rate may be within the normal range, around 60bpm to 100bpm. When Hashimoto's thyroiditis coincides with hypothyroidism, the heart rate may slow down, possibly dropping below 60bpm. Therefore, in Hashimoto's thyroiditis, as the thyroid function varies, so does the heart rate.